When Level Doesn't Look Level
Even when you do your level best, horizons that appear crooked are a common problem. And the solution isn't always as easy as it seems.
To "level set" readers, if your camera isn't level, your results just might reflect that. This may seem obvious, but in a rush to start shooting, such matters often get shortchanged. There are plenty of photos that get posted online with buildings leaning over, lake surfaces that look like someone is draining the pool, and whole forests standing tall but listing.
The traditional solution to keeping your camera level involves the use of a spirit/bubble level affixed atop the flash hot shoe. I would still consider this your best line of defense. Over the years, I've bought a quite a few of these little devices constructed from a small block of clear plastic and containing a bubble in the middle of a fluid-filled vial. Quality levels are precisely aligned with the camera body orientation. As you tilt the camera left or right, the bubble follows the camera movement back and forth. If the camera is level, the bubble will sit in the marked center of the fluid. Better versions have two levels at right angles to provide readings with the camera in both portrait and landscape mode.
There are also bubble versions with fluid vials in all three x, y and z axis. But a block of plastic big enough to hold three independent levels is quite a bit bigger than the standard 2-axis version. I've rarely found a need for the third. You can also find cheap versions with a single circular bubble level, but I've found them of questionable accuracy.
Most of these blocks of plastic have sharp corners, so if you carry one in your pocket like I do you'll either want to round the corners a bit or buy one that is already rounded some. Those sharp corners can hurt in your pocket.
You might wonder why you can't just line things up by eye and save on the bubble level, but this is harder than it seems. If your head is tilted at all when you compose a shot, things can be deceptive and appear level even when they're not. There are a lot of good shots possible in awkward spaces, and a bubble level lets you determine level regardless of other factors.
"Levelheaded" readers are probably wondering about the "artificial horizon" option included on many newer camera models. With a tilt indicator displayed directly in the viewfinder, it's tempting to forego the bubble level on top. But while this is indeed convenient, I still like the comfort of knowing at a glance that my camera is level. A slight change of composition might inadvertently result in an accidental tilt. I'll take all the help I can get and often use both.
On another level, there are times when a bubble won't give you the best results. Not everything in the natural world is itself level. Trees sometimes grow crooked. Lake shorelines curve and go where they do. Hillside slopes are, well, sloped. Any of these seen in real life may seem perfectly fine, but when photographed can appear crooked even when they aren't. Lacking any context outside the frame, all a viewer has to go on is what you choose to show them with your selected focal length and cropping. If your camera is level but things still look off, feel free to tweak as needed.
If you plan to shoot multiple images from the same spot, it helps to have your tripod head itself level, not merely the camera atop it. Panning at an angle complicates matters when shooting panoramas. While it is possible to level the panning base by adjusting the legs, several vendors make leveling bases that provide at least some room for adjustment even when the legs are slightly off. I wouldn't call one of these a necessity, but they sure can be nice to have. Consider having one as taking this whole "leveling" thing to the next level, so to speak.
Even when you do your level best, sometimes things end up looking crooked. When this happens, there's always Lightroom and Photoshop. Nothing beats having the ability to rotate an image until it looks its best. Just keep in mind that you'll end up losing pixels if you crop post-capture. Leveling an image before and after capture aren't on a level playing field, if you see my point.